Phytochemicals, nutrients & medications intended for transmission via the blood stream are typically entered into a person's blood stream through oral ingestion or syringe injection. An alternative form of treatment is the application of the phytochemicals, nutrients or medications to the skin with the medication being absorbed through the skin into the blood vessels underlying the epidermis part of the skin.
There are advantages of the skin application technique, i.e. the compounds can be applied to the place of the needed improvement of the structure, function or ailment and absorption into the blood vessels surrounding this condition provides a more direct and often a more effective treatment/repair of the structure, function or ailment. The problem with the technique is achieving penetration of the skin and specifically the stratum corneum area of the skin.
The compounds themselves are often not skin absorbable (evidence shows the only possible exceptions to this are sodium ions and water) and must be dissolved into a carrier or transport solution. There are solutions that can be made to be somewhat skin absorbent, especially when they are applied and then covered with a barrier such as plastic (example is Dow's Saran wrap) and this causes water to travel in and out of this upper layer of skin. This system is limited to "patches" already prepared, is expensive and limits the amount of ingredient that can be applied and the physical area that the plastic patch can be applied. Solvents are known that will achieve this in the presence of a "patch" but they must be at a precise ratio of ingredient to the solvent (near saturation point). Known solutions that are acceptable for topical application and used without a "patch" are either not sufficiently absorbable into the skin or not sufficiently solvent, e.g., if the ingredient is one having large molecules, the solution does not readily dissolve the medication.